(1) Field of the Invention
This invention relates to a dyo-sensitization-type photoelectric conversion element, which is used for a solar cell and other photoelectric conversion elements, an optical sensor, and a photo-revolving element.
(2) Description of the Related Art
Solar cells such as a silicon solar cell, a gallium arsenide (GaAs) solar cell, and an indlum phosphide (InP) solar cell have so far been put into practice. However, these cells are disadvantageously high-cost products.
A solar cell comprising an oxide semiconductor such as titanium dioxide (TiO.sub.2) is already known. However, titanium oxide exhibits poor photoelectric conversion since it absorbs exclusively light having a short wavelength. Thus, a dye-sensitization-type photoelectric conversion element has been proposed to improve photoelectric conversion.
A typical dye-sensitizatlon-type photoelectric conversion element has a laminate structure, illustrated in FIG. 1, which comprises a thin film 3 formed of titanium oxide particles, a transparent electrode 2 disposed on one side of the film 1, and a dye 4, an electrolyte 5, and an electrode 6 which are disposed in this order on the other side of the film 1. The dye absorbs sunlight to thereby generate electrons (e.sup.-) and holes (h.sup.+). The thus-released electrons (e.sup.-) are injected into the thin titanium oxide film 3, while the holes (h.sup.+) react With the electrolyte 5. When the employed electrolyte 5 includes, for example, iodide ions (I.sup.-) which react with the holes (h.sup.+) to thereby form I.sub.3.sup.- ions. EQU 3/2I.sup.- +h.sup.+.fwdarw.1/2I.sub.3.sup.-
On the electrode 6 disposed in the side of electrolyte 5, electrons (e.sup.-) react with I.sub.3.sup.- contained in the electrolyte, to thereby form I.sup.-. Thus, a circuit is formed to thereby generate an electromotive force. EQU 1/2I.sub.3.sup.- +e.sup.-.fwdarw.3/2I.sup.-
As specific examples of semiconductors which are used in such a photoelectric conversion element, there can be mentioned titanium oxide, zinc oxide, and zinc sulfide. Of these, titanium oxide is most popularly used. As specific examples of the dye, there can be mentioned ruthenium complexes and porphyrin derivatives.
Titanium oxide (titanium dioxide) includes three known crystal structures, i.e., anatase, brookite, and rutile. When titanium oxide is produced by a vapor phase method wherein titanium tetrachloride is subjected to premixed combustion by incorporation with oxygen or oxygen-containing gas, an anatase crystal structure is formed at a low temperature and remains stable. When the anatase crystal structure is heated for firing, a brookite crystal structure is formed at a temperature of 816 to 1.040.degree. C, and a rutile crystal structure is formed at a temperature higher than 1,040.degree. C. (Rikagaku jiten, 3rd edition, p. 514-515).
Although titanium oxide is most popularly used as a semiconductor employed for a dye-sensitization-type photoelectric conversion element, almost no investigation has been conducted on the influence of the particular types of crystal structure. Typically, rutile-type titanium oxide or anatase-type titanium oxide is employed, as described in Japanese Unexamined Patent Publication (kokai) No. H10-255863.